Conventionally, as one of tire air pressure detection apparatus, there has been a direct-type apparatus. In this type of the tire air pressure detection apparatus, transmitters provided with sensors such as pressure sensors are directly attached on the side of wheels to which tires are attached. In addition, an antenna and a receiver are provided on the side of a vehicle body. When a detection signal from the sensor is transmitted from the transmitter, the detection signal is received by the receiver through the antenna, and thus detection of the tire pressure is performed.
In such a direct-type tire air pressure detection apparatus, it needs to be determined whether the transmitted data is for the subject vehicle, and which wheel the transmitter that transmits the data is attached on. Therefore, pieces of ID information, for determining whether the data is for the subject vehicle or for the other vehicles, and for determining the wheel to which the transmitter is attached on, are given to the data transmitted from the corresponding transmitters.
In addition, in order to specify a position of the transmitter from the ID information included in the transmission data, the ID information of the transmitters needs to be registered in advance in the side of the receivers in association with the corresponding positions of the wheels. Therefore, at the time of tire rotation, relationships between the ID information of the transmitters and the positions of the wheels need to be reregistered to the receivers. Technologies that allow this registering to be automatically carried out have been proposed.
Specifically, in an apparatus described in Patent Literature 1, based on an acceleration detecting signal of an acceleration sensor provided in a transmitter on the wheel side, a wheel being set at a predetermined rotational position is detected, and the rotational position of the wheel at the time of receiving a radio signal from the transmitter is detected even on the vehicle body side. And the wheel position is specified by monitoring changes of a relative angle of these. In this method, a change of the relative angle of the rotational positions of the wheels detected on the wheel side and on the vehicle body side is monitored based on deviations of the predetermined number of data. By determining that the variability exceeds an allowable value in relation to an initial value, the wheel position is specified.